It is proposed to measure the dependence of lipid flexibility (or "lipid fluidity") on parameters such as temperature that can be experimentally varied. Our studies will concentrate on two systems that are particularly well-suited to our techniques and on which we have recently made preliminary measurements: (a) photoreceptor membranes, and (b) plasma membrane vesicles prepared from chick embryo fibroblasts. The reason for the choice of system (b) is that systematic comparisons can be made at various stages of transformation (by transforming agents such as Rous sarcoma virus) in order to characterize the effect of transformation on lipid flexibility. Our studies will also include model systems where appropriate. Particular emphasis will be placed on studies of the "purple membrane" from Halobacterium halobium, and important model system for studies of visual mechanisms. Two relatively new techniques will be brought to bear on the question of lipid flexibility: Raman spectroscopy and Fourier transform (F.T.) infrared spectroscopy. The same membrane preparations will be studied systematically using both the Raman and F.T. IR techniques. Contact with previous information obtained (on differing membrane preparations ) will be sought by making additional measurements on the same samples using the technique of fluorescence polarization.